Web former in a paper machine

Abstract: The invention concerns a twin-wire (10,20) web former in a paper machine, comprising a single-wire initial portion, after which the twin-wire zone starts in the area of the ribbed deck (22a) of the first forming shoe (22) placed inside one of the wire loops (10). After the first forming shoe (22), a pressure-loaded (p k ) dewatering unit (100) is fitted, which is provided with opposite sets of ribs (31,41) and with dewatering chambers (12,13). The first dewatering chamber (12) in this dewatering unit (100) is placed above the first forming shoe (22) so as, through its suction-deflector duct (16), to remove substantially water that drains from the area of the curved ribbed deck (22a) of the first forming shoe (22). The first dewatering chamber (12) is followed by a second dewatering chamber (13), whose lower part is connected with a set of dewatering and guide ribs (41), into whose gap spaces (41R) the drain duct (19) of the second dewatering chamber (13) is opened. Against the set of dewatering and guide ribs (41), a set of loading ribs operates, which is placed inside the opposite wire loop (20) and which operates by means of the medium pressure (p k ) of the loading hoses (33).

Description: The invention concerns a web former in a paper machine comprising a twin-wire zone formed by guiding a looped upper wire over a looped lower wire, wherein the web former comprises an initial, preferably substantially horizontal, single-wire portion preceding the twin-wire zone and being formed in connection with the looped lower wire, onto which initial single-wire portion a pulp jet is fed, the pulp jet being alternatively fed directly into a gap formed between the upper and lower wire loops with no initial single-wire portion in which case the twin-wire zone comprises an initial portion formed in connection with a forming roll over which the twin-wire zone is curved so that water is drained primarily through the lower wire loop. In the applicant's FI Patent No. 75,375 (equivalent to US Pats. Nos. 4,614,566 and 4,744,866), which constitutes one of the starting points of the present invention, a web former of a paper machine is described, in particular for modernizations of Fourdrinier wire parts. This former comprises a lower-wire loop placed in connection with the headbox of the paper machine and forming a single-wire, usually horizontal initial portion of the dewatering zone, in which zone water is drained by means of dewatering members out of the web that is being formed through the lower wire. The web former comprises an upper-wire unit, in which there is an upper wire guided by guide and web-forming rolls, which upper wire, together with the run of the lower wire, forms a twin-wire dewatering zone, which starts at the first forming roll, which is placed inside the loop of the upper wire and is provided with an open hollow face. After this, in the twin-wire dewatering zone, inside the loop of the lower wire, there is a forming shoe, which has a curved deck that guides the lower-wire loop and whose curve centres or centre are/is placed at the side of the lower-wire loop. After said forming shoe, inside the loop of the lower wire, there is a forming roll, which guides the twin-wire forming zone and on whose guide sector the run of the twin-wire forming zone is curved downwards. Substantially the whole of the twin-wire forming zone in accordance with said FI Pat. 75,375 is placed above the level determined by the single-wire initial portion, the twin-wire forming zone being curved upwards from said plane on a sector of the first forming roll. After the initial dewatering has taken place to a suitable extent through the lower wire in the single-wire initial portion, the dewatering takes place in the twin-wire dewatering zone initially on said sector of the first open forming roll in two directions through both of the wires. Hereupon, in the area of the next forming shoe, the dewatering takes place primarily upwards through the upper wire, after which the dewatering pressure is increased further in the area of the second forming roll, while the dewatering still takes place primarily through the upper wire. The formers described in said FI Patent 75,375 and some other, corresponding formers have been marketed by the applicant during about 10 years under the trade marks "Sym-Former" and "Sym-Former R". From these formers, in several different applications, a considerable amount of operating experience has been obtained, which experience has provided the background and the starting point for the present invention. With increased running speeds of paper machines, several problems in the web formation have been manifested with increased emphasis. In the former section of a paper machine, the phenomena that act upon the fibre mesh and upon the water, which is still relatively free in connection with the fibre mesh, such as centrifugal forces, are, as a rule, increased in proportion to the second power of the web speed. The highest web speeds of some existing paper machines are already of an order of 1500 metres per minute. However, machines are being planned in which a web speed of about 2000 m/min or even higher web speeds are aimed at. With respect to the prior art related to the present invention, reference is also made to the US Patent No. 4,769,111 granted to Messrs. A. Ahlstrom Corporation and to the applicant's FI Pat. No. 84638, in which formers marketed under the applicant's trade mark "MB-former" are described. Further, reference is made to the applicant's FI Patent Nos. 91788, 93032, and 90673, in which various combination concepts of said MB formers and of hybrid and gap formers are described. The object of the present invention is further development of the applications of the prior-art MB units as well as to provide novel advantageous combinations of said MB units and of hybrid formers. It is a principal object of the invention to provide a combination of a hybrid former and a MB former, by whose means it is possible to achieve good base formation and formation of the web while maintaining a satisfactory retention level also at increased web speeds. A particular object of the invention is further development of the formers described in said FI Patent 75,375 and of corresponding "Sym-Former" and "Sym-Former R" formers so that their running speeds can be increased, e.g. in connection with modernizations, to a considerably higher level, typically to a speed range of 1200...1500 metres per minute, so that neither the dewatering capacity nor the formation of the web accomplished in the former constitutes an obstacle for an increased speed. It is a further object of the invention to provide a former in which, if necessary, the increased dewatering capacity can be utilized, besides a higher web speed, also as a thicker grade produced and/or as a lower consistency in the headbox, which latter feature already in itself improves many properties of the paper. In view of achieving the objectives stated above and those that will come out later, the web former according to the invention is mainly characterized by the features recited in claim 1 of the two appended sets of claims. Further features of the web former are defined in the dependent claims. EP-A-0 552 139, which falls within the terms of Article 54(3) EPC for the designated contracting states AT, DE, FR, GB, IT and SE, discloses a hybrid former according to claim 1 of the set of claims for the designated contracting states CH and LI. According to the invention, when a pressure-loaded MB twin-wire zone and a preceding forming shoe that is fitted inside the lower wire and provided with a curved ribbed deck in the initial portion of the twin-wire zone of the former are employed, the dewatering capacity can be increased and, which is also important, the formation of the web can be improved while the retention remains at a satisfactory level. In a preferred embodiment of the invention, the MB zone is preceded by a dewatering chamber fitted above the first forming shoe, and said chamber is directly followed by a second dewatering chamber fitted above the MB zone. Out of the chambers, the waters are removed through drain ducts, which are preferably placed at the driving side of the machine only. Said dewatering chambers are placed in the space between the first guide roll, placed inside the loop of the upper wire, and the first forming roll. On the first forming roll, the twin-wire zone is curved after the MB zone upwards, whereupon there are preferably dewatering members in accordance with the "Sym-Former" concept, such as a second forming shoe and a second forming roll placed inside the loop of the lower wire. When the MB zone in accordance with the invention is reached, the dry solids content of the web is preferably in the range of k 1 ≈ 1.0...3 %, preferably in the range of k 1 ≈ 1.2...1.8 %, in which case the loading ribs in the MB zone produce an efficient dewatering that improves the formation and that takes place both through the upper wire and through the lower wire. In the invention, symmetric dewatering is also accomplished so that the structure of the web in the z-direction can also be made sufficiently symmetric in respect of the distribution of fillers and fines. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematic side view of a hybrid-MB former combination in accordance with the present invention. Figure 2 is a schematic vertical sectional view in the machine direction of the initial part of the twin-wire zone in a hybrid former in accordance with the invention. Figure 3 is a detailed vertical sectional view in the machine direction of a twin-wire MB zone as shown in Figs. 1 and 2. Figure 4 is a vertical sectional view in the machine direction and on an enlarged scale of the detail DET. encircled by the dashed-dotted line in Fig. 3. The hybrid former shown in Figs. 1 and 2 comprises a lower wire 20 guided by the guide rolls 21. The pulp suspension jet J is fed out of the discharge opening 53 of the headbox onto the single-wire initial portion 20a of the lower wire 20. The paper web W 0 that is formed on this initial portion 20a has time to reach a certain degree of couching on its face placed against the lower wire 20 before the twin-wire zone starts. The run of the lower wire 20 is guided outside the twin-wire zone by the breast roll 21a and by the guide rolls 21. Further, Fig. 1 shows the frame part 65 placed inside the loop of the lower wire 20, which frame is, for example, a part of the frame of an existing Fourdrinier wire part, which has been modernized to make a twin-wire hybrid former in accordance with the invention. The former includes an upper-wire unit 60, which includes the frame part 61. The twin-wire forming zone starts at the first forming shoe 22 and ends at the guide roll 11c of the upper wire 10, after which the web W follows the lower wire 20. The run of the upper wire is guided outside the twin-wire zone by the first guide roll 11a and by the latter guide roll 11c as well as by the upper guide rolls 11d. The former shown in Fig. 1 is typically a "Sym-Former R" modernized from an existing Fourdrinier former, which has been provided with a MB unit 100 placed and fitted, in accordance with the invention, at the beginning of the twin-wire zone in order to increase the dewatering capacity and/or to improve the web W formation. According to Figs. 1 to 3, the guide roll 11a of the upper wire 10 guides the upper wire onto the web W in the area of the ribbed deck 22a of the first forming shoe 22, preferably approximately at its fourth rib when the ribbed deck 22a comprises about 7 ribs. The ribbed deck 22a of the forming shoe 22, which is placed inside the loop of the lower wire 20, has a relatively large curve radius R 1 , which is, as a rule, dimensioned in the range of R 1 ≈ 3...12 m, preferably in the range of R 1 ≈ 5...8 m. The open gap spaces in the ribbed deck 22a, which are placed against the inner face of the lower wire 20, can be connected to a source of negative pressure through the frame box of the forming shoe 22, which is illustrated by the arrow p 0 . In the area of the forming shoe 22, the dewatering takes place mainly through the upper wire 10 by the effect of the tensioning pressure p = T/R 1 (T = tension of the wire 10) of the upper wire 10, which dewatering is promoted by the field of centrifugal force. The dry solids content k 1 of the web W is, at the beginning of the twin-wire zone, of an order of k 1 ≈ 1.0...3 %, preferably in the range of k 1 ≈ 1.2...1.8 %. According to Figs. 1 to 3, the first forming shoe 22 is followed by a MB unit 100 fitted in accordance with the invention. The MB unit 100 comprises a first dewatering chamber 12 fitted inside the loop of the upper wire 10 and, immediately after said chamber, a second dewatering chamber 13. Said chambers 12 and 13 communicate with a source 50 of negative pressure so that the negative pressures p 1 and p 2 present inside the chambers 12 and 13 can be set or adjusted individually. The first chamber 12 is provided with a water drain duct 12f, which is preferably placed at the driving side of the machine only. In a corresponding way, the second dewatering chamber 13 is provided with a corresponding water drain duct 13b. The first chamber 12 is attached to the frame parts 61 of the upper-wire unit 60 by means of support members 12a, and similarly the second chamber 13 is attached to the frame parts 61 of the upper-wire unit 60 by means of its support members 13a, if necessary, so that its position can be set or adjusted. A more detailed exemplifying embodiment of the twin-wire zone of the MB unit 100, which comes out from Figs. 3 and 4, will be described in more detail later. According to Fig. 1, the MB unit 100, which has been fitted in accordance with the invention, is followed by a "Sym-Former" twin-wire zone similar to what is known from the prior art. After the MB unit 100, the twin-wire zone is guided by the smooth-faced 14' forming roll 14, on whose sector a the twin-wire zone is curved upwards. The magnitude of the sector a is, as a rule, chosen as a ≈ 20°...45°. After this, a second forming shoe 24 follows, which has a ribbed deck 24a, which may communicate with negative pressure if necessary. The curve radius R 2 of the ribbed deck 24a of the forming shoe 24 is, as a rule, chosen in the range of R 2 ≈ 2...6 m. The second forming shoe 24 is followed by a forming roll 25 placed inside the loop of the lower wire 20, the twin-wire zone being curved downwards on the sector b ≈ 30°...50° of said forming roll 25. The forming roll 25 is preferably smooth-faced 25' and has no suction zone. At the vicinity of the forming roll 24, inside the upper-wire loop 10, there is a water drain box 15 and a water drain doctor 15a passing into said box. After this, according to Fig. 1, the twin-wire zone runs as downwards inclined onto the guide or forming roll 11b placed inside the loop of the upper wire 10, which roll returns the twin-wire zone to the plane T-T determined by the single-wire initial portion 21 or to the proximity of said plane. After this, a pick-up device in itself known follows, which comprises a pick-up fabric 52 and a pick-up roll 51 provided with a suction zone 51a, on which roll the web W is separated from the lower wire 20 and is transferred into the press section of the paper machine (not shown). In the following, mainly with reference to Fig. 3, a preferred exemplifying embodiment of the construction of a twin-wire MB zone in accordance with the invention will be described. Properly speaking, the twin-wire MB zone starts after the last forming rib 22a 0 of the first forming shoe 22, being guided by the first pair of ribs 31a of the loading unit 23. After the latter rib in said pair of ribs 31a, in the twin-wire zone, there follows the first fixed dewatering and guide rib 41a, which operates against the inner face of the upper-wire loop 10. The first rib 41a is followed by equivalent dewatering and guide ribs 41 as uniformly spaced. The last one of said ribs is denoted with the reference 41b. According to Fig. 3, opposite to the fixed dewatering and guide ribs 41a,41,41b, pairs of loading ribs 31b and 31c operate, which are placed facing the gap spaces 41R between said dewatering and loading ribs and which are placed inside the lower-wire loop 20. The width a 1 of the front face of the dewatering and guide ribs 41 in the machine direction is preferably measured in the range of a 1 ≈ 50...60 mm, and the width b 1 of the gap space between the ribs 31 and 41 in the machine direction, which space has no rubbing face, is preferably dimensioned in the range of b 1 ≈ 10...15 mm. Said pairs of ribs 31b,31c as well as the first pair of loading ribs 31a that precedes them are connected in pairs, by means of connecting parts 35, to W-section loading members 34, in whose interior pairs of loading hoses 33 operate. By means of the pressures p k of the pressure medium that is passed into these pairs of hoses, the pressure is regulated with which the pairs of loading ribs 31a,31b,31c are pressed against the inner face of the lower-wire loop 20. The back-up supports for this loading consist of the dewatering and guide ribs 41a,41,41b, which are fixed. By means of the loading of the ribs 31a,31b,31c, the compression pressure between the wires 10,20 is regulated which removes water out of the web W and which is, in the MB zone, applied to the web W placed between the wires 10,20. The loading pressures p k between the different pairs of ribs 31a,31b,31c can be regulated separately in order to "profile" the loading in the running direction of the wires 10,20. The dewatering pressure can also be profiled in the transverse direction by arranging the loading hoses 33 so that they can be loaded with different pressures in the transverse direction by means of blocks. The box of the loading unit 23 may communicate with negative pressure, which is illustrated by the arrow p 3 , said negative pressure having a dewatering effect through the lower wire 20 through the gap spaces 31R between the loading ribs 31. In the following, with reference to Figs. 2,3 and 4, the construction and the operation of the dewatering chambers 12 and 13 will be described in more detail. The lower wall 12c of the first dewatering chamber 12 defines a gap space 12b above the ribbed deck 22a of the first forming shoe 22 and above the first pair of loading ribs 31a. Through this gap space 12b, water is drained that is derived mainly from the area of the ribbed deck 22a of the first forming shoe 22 as well as water that has been "doctored" by the first fixed dewatering and guide rib 41a from the inner face of the upper wire 10. These waters are passed into the first dewatering box 12 in the direction of the arrow F 1 through the drain duct 16. Said duct 16 is placed in the space between the rear wall 12c of the first chamber 12 and the vertical wall 13c of the second chamber 13. The duct 16 forms a vertical suction-deflector duct, through which the water is removed in the direction of the arrow F 1 into the first chamber 12, being aided by the negative pressure p 1 present in said chamber. Out of the first chamber 12, the water is drained through its drain duct 12f, which is preferably placed at the driving side of the machine only. In connection with the lower wall of the second dewatering chamber 13, fixed dewatering and guide ribs 41 are placed, of which ribs the first rib 41a is fixed to the transverse rib 13d interconnecting the lower edges of the wall portions 13c and 18a by means of a dovetail joint 13e,14c (Fig. 4) or equivalent. Above the ribs 41, there is a curved wall 18a, which defines the drain duct 19 of the second chamber 13 together with the curved outer wall 18b, the last rib 41b being placed in connection with the lower edge of the latter wall 18b. Water is drained in the direction of the arrow F 2 through the gap spaces 41R between the ribs 41 along the duct 19 into the second chamber 13, being aided by the negative pressure p 2 present in said chamber. The water is drained further out of the chamber 13 through its drain duct 13b, which is preferably placed at the driving side of the machine only. In the loading unit 23, after said loading ribs 31c, a fixed pair of loading ribs 31d is placed. This pair of ribs 31d, together with the last rib 22a 0 of the first forming shoe 22, define the inlet and outlet directions of the twin-wire zone. In the area between said ribs 22a 0 and 31d, the twin-wire zone runs along a gently meandering path when loaded by the loading ribs 31 against the fixed ribs 41. In such a case, the ribs 31 and 41, which operate alternatingly against one another, apply dewatering impulses to the web W, which impulses improve the formation and produce dewatering, to some extent through the lower wire 20 and especially through the upper wire 10, if necessary, aided by negative pressures p 1 ,p 2 ,p 3 . The ribs 31a,31b,31c,31d,41,41a,41b have been made as replaceable ribs that extend across the entire width of the wires 10,20, which ribs are either of a wear-resistant material, for example of a ceramic, throughout, or which ribs are provided with a wear-resistant front face, which operates against the inner faces of the wires 10,20, while the water drained out of the web W acts as a lubricant that reduces the wear and the friction. The same also applies in respect of the ribs 22a and 24a in the decks of the forming shoes 22 and 24. After the MB zone, the dry solids content k 2 of the web W is, as a rule, of an order of k 2 ≈ 3...6 %, preferably in the range of k 2 ≈ 4...5 %. After the second forming roll 25, the dry solids content k 3 of the web is typically of an order of k 3 ≈ 6...12 %, preferably in the range of k 3 ≈ 8...10 %. After the twin-wire zone, at the guide roll 11c of the upper wire 10, the dry solids content k 4 is typically of an order of k 4 ≈ 8...14 %, preferably in the range of k 4 ≈ 10...12 %. In the following, with reference to Fig. 4, a preferred exemplifying embodiment will be described of the lower part of the drain duct 16 of the first dewatering chamber 12. The dimensions and the shape of the throat K of the drain duct 16 of the chamber 12, placed at and above the first rib 41a, are highly critical depending on the amounts of water to be drained, on the speed of the machine, and on the paper grade produced. As is known from prior art, attempts have been made to regulate the width of said throat K by adjusting the position of the first dewatering chamber 12 by means of various lifting mechanisms, but these devices are expensive, complicated, and they have not been able to provide a solution satisfactory under all circumstances, in particular not in respect of the shape of the throat K. According to Fig. 1, these problems have been solved so that the fixed curved wall 17 in Fig. 3 has been replaced by a replaceable form rib 17a, which is preferably made of plastic. Said rib 17a is fixed to a fastening base 12e provided in connection with the plane lower wall 12d of the first box 12 by means of the dovetail groove 17b in the rib 17a. Said ribs 17a are made and kept in reserve as of different measures and different profile 17c of the outer face, so that, by replacing the ribs 17a, it is possible to set the width and the profile of the throat K in the draining direction F 1 and, if necessary, also in the transverse direction optimally. By means of replaceable ribs 17a, the construction can be made operative with different machine speeds, both in slower machines, in which the water follows the face of the upper wire 10, and in machines of higher speeds, in which the water moving in the space 12b rises apart from the inner face of the upper wire 10. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above for the sake of example only.

Claims: Claims for the following Contracting States : AT, DE, FR, GB, IT, SE Web former in a paper machine, said former comprising a twin-wire zone formed by guiding a looped upper wire (10) over a looped lower wire (20), which twin-wire zone starts in the area or at the proximity of the curved ribbed deck (22a) of a forming shoe (22) placed inside the lower wire loop (20), wherein said former comprises an initial, preferably substantially horizontal, single-wire portion (20a) preceding the twin-wire zone and being formed in connection with the looped lower wire (20), onto which initial single-wire portion (20a) a pulp jet (J) is fed, the pulp jet being alternatively fed directly into a gap formed between the upper and lower wires with no initial single-wire portion in which case the twin-wire zone comprises an initial portion formed in connection with a forming roll over which the twin-wire zone is curved so that water is drained primarily through the lower wire loop, wherein said former further comprises a pressure-loaded dewatering unit (100) in the area of said twin-wire zone, said pressure-loaded dewatering unit (100) comprising: a first dewatering chamber (12) placed above said forming shoe (22) inside the upper wire loop (10) and provided with a lower wall (12c) defining a gap space (12b) above said deck (22a), through which gap space (12b) water that is derived mainly from the area of said deck (22a) is drained and passed into the first dewatering chamber (12) through a vertical suction-deflector duct (16), a second dewatering chamber (13) following said first dewatering chamber (12) inside said upper wire loop (10), a set of dewatering and guide ribs (41a, 41, 41b) connected to the lower part of the second dewatering chamber (13), wherein the first dewatering and guide rib (41a) is fixed and operates as a deflector rib at the throat (K) in the lower end of the suction-deflector duct (16), water being drained through the gap spaces (41R) between said dewatering and guide ribs (41a, 41, 41b) and passed into the second dewatering chamber (13) through a drain duct (19) opening into said gap spaces (41R), a loading unit (23) placed inside the lower wire loop (20) after the forming shoe (22) and provided with loading ribs (31a, 31b, 31c), which are pressed against the lower wire (20), preferably by means of loading hoses (33) into which a pressure medium is passed, wherein the first loading rib or pair of loading ribs (31a) is fitted underneath the first dewatering chamber (12) before said first dewatering and guide rib (41a), and wherein said first loading rib or pair of loading ribs (31a) is followed by a set of loading ribs (31b, 31c) which operates against said set of dewatering and guide ribs (41a, 41, 41b). Web former as claimed in claim 1, characterized in that said dewatering and guide ribs (41a, 41, 41b) are all fixed ribs, that preferably 4 to 7 dewatering and guide ribs are provided, and that, after the last rib (41b) of said set of dewatering and guide ribs, one or several fixed guide ribs (31b) are provided, which are placed inside the lower wire loop (20) and fixed to a frame part of the loading unit (23). Web former as claimed in claim 1 or 2, characterized in that each of said first and second dewatering chamber (12,13) communicates with a source (50) of negative pressure so that the negative pressure (p 1 ,p 2 ) present in said dewatering chambers (12,13) can be set and/or regulated independently and that said dewatering chambers (12,13) communicate with second drain ducts (12f,13b) preferably at the driving side of the machine only. Web former as claimed in any of claims 1 to 3, characterized in that said forming shoe (22) and/or a frame box of the loading unit (23) communicate(s) with a source of negative pressure (P 0 ,P 3 ). Web former as claimed in any of the claims 1 to 4, characterized in that the throat part (K) placed at the lower end of the suction-deflector duct (16) has been arranged so that its dimensioning and/or profile can be adjusted, said throat part being provided with a replaceable form rib (17a), which constitutes a wall that defines said throat part primarily from above, so that the measures and the shape of said throat part (K) can be set by replacing said form rib (17a) with a different form rib so as to change the measures and the shape of the curved outer face (17c) of said form rib. Web former as claimed in claim 5, characterized in that said replaceable form rib (17a) is attached to the outer face of the lower wall, preferably a plane wall (12d), of the first dewatering chamber (12) by means of a groove-projection joint (12e, 17b) or an equivalent snap joint. Web former as claimed in any of the claims 1 to 6, characterized in that, before the twin-wire zone, there is a single-wire forming zone (20a), which is preferably substantially horizontal, that the twin-wire zone is curved down from the plane (T-T) of the single-wire initial portion (20a) on the curved ribbed deck (22a) of the forming shoe (22) and runs further as downwards inclined, being guided by the pressure-loaded ribbed zone of the dewatering unit (100), after which, in the twin-wire zone, there follows a forming roll (14), which is placed inside the upper-wire loop (10), which is preferably provided with a smooth solid face (14'), and on which the twin-wire zone is curved into an upwards inclined run, on which there is a second forming shoe (24) inside the lower-wire loop (20), the twin-wire zone being curved in the area of the ribbed deck (24a) of the second forming shoe (24) with a large curve radius (R 2 ) in the opposite direction, compared with the preceding curve zone (a), and that said second forming shoe (24) is followed by a second forming roll (25), which is placed inside the lower-wire loop (20) and on which the twin-wire zone is curved into a downwards inclined run with a curve radius (R 3 ) considerably smaller than the curve radius (R 2 ) of the preceding zone. Web former as claimed in claim 7, characterized in that, after said second forming roll (25), a forming or guide roll (11b) placed inside the upper-wire loop (10) guides the twin-wire zone in or near the plane (T-T) determined by the single-wire initial portion (20a), and that after this the web (W) is guided to follow the lower wire (20), from which the web (W) is separated on the suction zone (51a) of a pick-up roll (51) and is transferred on a pick-up fabric (52) into the press section. Web former as claimed in any of the claims 1 to 8, characterized in that, as the web (W) arrives at the beginning of the twin-wire zone in the case where an initial single-wire portion (20a) is provided or as the web leaves the forming roll in the initial portion of the twin-wire zone in the case where no initial single-wire portion is provided, its dry solids content is within the range of about 1% to about 3%, preferably within the range of about 1.2% to about 1.8%, and, as the web (W) departs from the twin-wire zone, its dry solid content is within the range of about 8% to about 14%, preferably within the range of about 10% to about 12%. Claims for the following Contracting States : CH, LI Web former in a paper machine, said former comprising a twin-wire zone formed by guiding a looped upper wire (10) over a looped lower wire (20), which twin-wire zone starts in the area or at the proximity of the curved ribbed deck (22a) of a forming shoe (22) placed inside the lower wire loop (20), wherein said former comprises an initial, preferably substantially horizontal, single-wire portion (20a) preceding the twin-wire zone and being formed in connection with the looped lower wire (20), onto which initial single-wire portion (20a) a pulp jet (J) is fed, the pulp jet being alternatively fed directly into a gap formed between the upper and lower wires with no initial single-wire portion in which case the twin-wire zone comprises an initial portion formed in connection with a forming roll over which the twin-wire zone is curved so that water is drained primarily through the lower wire loop, wherein said former further comprises a pressure-loaded dewatering unit (100) in the area of said twin-wire zone, said pressure-loaded dewatering unit (100) comprising: a first dewatering chamber (12) placed above said forming shoe (22) inside the upper wire loop (10) and provided with a lower wall (12c) defining a gap space (12b) above said deck (22a), through which gap space (12b) water that is derived mainly from the area of said deck (22a) is drained and passed into the first dewatering chamber (12) through a vertical suction-deflector duct (16), a second dewatering chamber (13) following said first dewatering chamber (12) inside said upper wire loop (10), a set of dewatering and guide ribs (41a, 41, 41b) connected to the lower part of the second dewatering chamber (13), water being drained through the gap spaces (41R) between said dewatering and guide ribs (41a, 41, 41b) and passed into the second dewatering chamber (13) through a drain duct (19) opening into said gap spaces (41R), and a loading unit (23) placed inside the lower wire loop (20) after the forming shoe (22) and provided with loading ribs (31a, 31b, 31c), which are pressed against the lower wire (20), preferably by means of loading hoses (33) into which a pressure medium is passed, wherein a set of said loading ribs (31b, 31c) operates against said set of dewatering and guide ribs (41a, 41, 41b). Web former as claimed in claim 1, characterized in that the first rib or pair of ribs (31a) of the loading unit (23) is fitted underneath the first dewatering chamber (12) before the first dewatering and guide rib (41a), and that said first dewatering and guide rib (41a) is fixed and operates as a deflector rib at the throat (K) in the lower end of the suction-deflector duct (16). Web former as claimed in claim 1 or 2, characterized in that said dewatering and guide ribs (41a, 41, 41b) are all fixed ribs, that preferably 4 to 7 dewatering and guide ribs are provided, and that, after the last rib (41b) of said set of dewatering and guide ribs, one or several fixed guide ribs (31b) are provided, which are placed inside the lower wire loop (20) and fixed to a frame part of the loading unit (23). Web former as claimed in any of the claims 1 to 3, characterized in that each of said first and second dewatering chamber (12,13) communicates with a source (50) of negative pressure so that the negative pressure (p 1 ,p 2 ) present in said dewatering chambers (12,13) can be set and/or regulated independently and that said dewatering chambers (12,13) communicate with second drain ducts (12f,13b) preferably at the driving side of the machine only. Web former as claimed in any of the claims 1 to 4, characterized in that said forming shoe (22) and/or a frame box of the loading unit (23) communicate(s) with a source of negative pressure (p 0 ,p 3 ). Web former as claimed in any of the claims 1 to 5, characterized in that the throat part (K) placed at the lower end of the suction-deflector duct (16) has been arranged so that its dimensioning and/or profile can be adjusted, said throat part being provided with a replaceable form rib (17a), which constitutes a wall that defines said throat part primarily from above, so that the measures and the shape of said throat part (K) can be set by replacing said form rib (17a) with a different form rib so as to change the measures and the shape of the curved outer face (17c) of said form rib. Web former as claimed in claim 6, characterized in that said replaceable form rib (17a) is attached to the outer face of the lower wall, preferably a plane wall (12d), of the first dewatering chamber (12) by means of a groove-projection joint (12e,17b) or an equivalent snap joint. Web former as claimed in any of the claims 1 to 7, characterized in that, before the twin-wire zone, there is a single-wire forming zone (20a), which is preferably substantially horizontal, that the twin-wire zone is curved down from the plane (T-T) of the single-wire initial portion (20a) on the curved ribbed deck (22a) of the forming shoe (22) and runs further as downwards inclined, being guided by the pressure-loaded ribbed zone of the dewatering unit (100), after which, in the twin-wire zone, there follows a forming roll (14), which is placed inside the upper-wire loop (10), which is preferably provided with a smooth solid face (14'), and on which the twin-wire zone is curved into an upwards inclined run, on which there is a second forming shoe (24) inside the lower-wire loop (20), the twin-wire zone being curved in the area of the ribbed deck (24a) of the second forming shoe (24) with a large curve radius (R 2 ) in the opposite direction, compared with the preceding curve zone (a), and that said second forming shoe (24) is followed by a second forming roll (25), which is placed inside the lower-wire loop (20) and on which the twin-wire zone is curved into a downwards inclined run with a curve radius (R 3 ) considerably smaller than the curve radius (R 2 ) of the preceding zone. Web former as claimed in claim 8, characterized in that, after said second forming roll (25), a forming or guide roll (11b) placed inside the upper-wire loop (10) guides the twin-wire zone in or near the plane (T-T) determined by the single-wire initial portion (20a), and that after this the web (W) is guided to follow the lower wire (20), from which the web (W) is separated on the suction zone (51a) of a pick-up roll (51) and is transferred on a pick-up fabric (52) into the press section. Web former as claimed in any of the claims 1 to 9, characterized in that, as the web (W) arrives at the beginning of the twin-wire zone in the case where an initial single-wire portion (20a) is provided or as the web leaves the forming roll in the initial portion of the twin-wire zone in the case where no initial single-wire portion is provided, its dry solids content is within the range of about 1% to about 3%, preferably within the range of about 1.2% to about 1.8%, and, as the web (W) departs from the twin-wire zone, its dry solid content is within the range of about 8% to about 14%, preferably within the range of about 10% to about 12%.

Inventors: JAAKKOLA JYRKI, KIVIRANTA SEPPO, LINSURI ARI, ODELL MICHAEL, POIKOLAINEN ANTTI, SALMINEN SAMPPA, JAAKKOLA, JYRKI, KIVIRANTA, SEPPO, LINSURI, ARI, ODELL, MICHAEL, POIKOLAINEN, ANTTI, SALMINEN, SAMPPA
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1994
Main classification: D21F9
IPC codes: D21F9, D21F1